It works like this:
1. Posting work cost 25 credits(you can post only 2 works a day, after that price will increase).
2. Every user can vote for your work only once.
3. Every 10 votes you will get credits(amount of credits will be equal to rating*10)
4. You also get 1 credit for voting (2 for voting in challenges)
5. If you want you can delete comments but it cost 10 credits.
6. You can add users to your Friend list.
7. If you liked the comment someone has written for you, give him or her credits!
8. Have fun and be friendly :)
9. If you see a bug report it, if you will be the first one - you will get 100 credits
Some other articles about features of this website (I strongly recommend reading F.A.Q.):